Operational amplifier with class ab output

ABSTRACT

An operational amplifier includes an output stage, an input stage, a first auxiliary amplifier, and a second auxiliary amplifier. The output stage includes a first output transistor and a second output transistor. The input stage is configured to drive the output stage. The first auxiliary amplifier is coupled to an output of the input stage and to an input of the first output transistor. The first auxiliary amplifier is configured to bias the first output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the first output transistor. The second auxiliary amplifier is coupled to the output of the input stage and to an input of the second output transistor. The second auxiliary amplifier is configured to bias the second output transistor for class AB operation, and to isolate the input stage from a bias voltage applied to the second output transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/202,606, filed Aug. 7, 2015, titled “Novel Class ABOutput Biasing Scheme for Operational Amplifier,” which is herebyincorporated herein by reference in its entirety.

BACKGROUND

Of the many available electronic devices, operational amplifiers(op-amps) are some of the most widely used. Op-amps are efficient andversatile devices that can be used in a variety of applications, such assignal conditioning, analog instrumentation, analog computation, etc.

An op-amp may employ one of several different circuit arrangements. Inone example, a class A amplifier reproduces an entire input signalbecause an active element of the class A amplifier, such as atransistor, is constantly in the active mode. Class A amplifierstypically have high power consumption because the active elementconstantly conducts current.

In another example, a class B amplifier typically employs twocomplementary output transistors, with each output transistor beingturned on for half of the time and turned off for the other half of thetime. That is, one output transistor operates as a current source, andthe other output transistor operates as a current sink. Thisconfiguration is sometimes referred to as a “push-pull” configurationsince a first branch of the output stage “pushes” or sources currents toa load while a second branch of the output stage “pulls” or sinkscurrent from the load. A class B amplifier has lower power consumptionthan a class A amplifier but may be susceptible to crossover distortiondue to the turn-on of one output transistor not matching the turn-off ofthe other output transistor.

A class AB amplifier also employs two complementary output transistors(similar to a class B amplifier). Class AB amplifiers avoid the highpower consumption of a class A amplifier by always having one outputbranch substantially turn off when the other output branch is turned on.Although the current in one leg of a class AB amplifier is substantiallyturned off there is a small amount of current flowing in that leg. Thesmall residual current in the class AB amplifier avoids the crossoverdistortion produced the turning on and off of the currents in class Bamplifiers. Thus class AB amplifiers are able to achieve a relativelyhigh current output while maintaining a low quiescent current. Thecurrents in class AB amplifiers are inversely related such that when onecurrent becomes large, the other current becomes very small.

SUMMARY

Operational amplifiers (op-amps) with class AB output biasing circuitrythat increases headroom in a stage preceding the output stage aredisclosed herein. In one embodiment, an operational amplifier includesan output stage, an input stage, a first auxiliary amplifier, and asecond auxiliary amplifier. The output stage includes a first outputtransistor and a second output transistor. The input stage is configuredto drive the output stage. The first auxiliary amplifier is coupled toan output of the input stage and to an input of the first outputtransistor. The first auxiliary amplifier is configured to bias thefirst output transistor for class AB operation and to isolate the inputstage from a bias voltage applied to the first output transistor. Thesecond auxiliary amplifier is coupled to the output of the input stageand to an input of the second output transistor. The second auxiliaryamplifier is configured to bias the second output transistor for classAB operation, and to isolate the input stage from a bias voltage appliedto the second output transistor.

In another embodiment, a multi-stage operational amplifier includes afirst stage, a second stage, a first auxiliary amplifier, and a secondauxiliary amplifier. The first stage is configured to amplify an inputvoltage. The second stage is configured to drive an output terminal ofthe two-stage operational amplifier. The second stage is coupled to anoutput of the first stage. The first auxiliary amplifier is coupled tothe output of the input stage and to a first input of the second stage.The first auxiliary amplifier is configured to generate a first biasvoltage and apply the first bias voltage to the second stage for classAB operation. The first auxiliary amplifier also isolates the firststage from the first bias voltage. The second auxiliary amplifier iscoupled to the output of the input stage and to a second input of thesecond stage. The second auxiliary amplifier is configured to generate asecond bias voltage and apply the second bias voltage to the secondstage for class AB operation. The second auxiliary amplifier alsoisolates the first stage from the second bias voltage.

In a further embodiment, a class AB output operational amplifierincludes an output stage, an input stage, a first auxiliary amplifier,and a second auxiliary amplifier. The output stage includes a high-sideoutput transistor and a low-side output transistor. The input stage isconfigured to drive the output stage. The first auxiliary amplifier iscoupled to an output of the input stage and to an input of the low-sideoutput transistor. The first auxiliary amplifier is configured to biasthe low-side output transistor for class AB operation, and to isolatethe input stage from a bias voltage applied to the low-side outputtransistor. The first auxiliary amplifier includes a first transistorand a second transistor coupled as a differential pair. An inputterminal of the first transistor is coupled to the output of the inputstage. An input terminal of the second transistor is connected to apredetermined DC voltage. The second auxiliary amplifier is coupled tothe output of the input stage and an input of the high-side outputtransistor. The second auxiliary amplifier is configured to bias thehigh-side output transistor for class AB operation, and to isolate theinput stage from a bias voltage applied to the high-side outputtransistor. The second auxiliary amplifier includes a third transistorand a fourth transistor coupled as a differential pair. An inputterminal of the third transistor is coupled to the output of the inputstage. An input terminal of the fourth transistor is connected to thepredetermined DC voltage. Isolation of the input stage from the outputstage by the first auxiliary amplifier and the second auxiliaryamplifier provides the input stage with headroom of approximatelyone-half of the power supply voltage that powers the operationalamplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a schematic diagram for a class AB output operationalamplifier (op-amp) in accordance with various embodiments;

FIG. 2 shows a schematic diagram for a low-side output transistorbiasing circuit in accordance with various embodiments; and

FIG. 3 shows a schematic diagram for a low-side output transistorbiasing circuit in accordance with various embodiments;

FIG. 4 shows a schematic diagram for compensation of a class AB outputop-amp in accordance with various embodiments;

FIGS. 5 and 6 show diagrams of models of a class AB op-amp in accordancewith various embodiments used for pole-zero analysis.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, different companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . .” Also, the term “couple” or “couples” isintended to mean either an indirect or direct wired or wirelessconnection. Thus, if a first device couples to a second device, thatconnection may be through a direct connection or through an indirectconnection via other devices and connections. The term “approximately”is intended to mean within +1-10% of a stated value.

In class AB operational amplifiers (op-amps), the output transistors arebiased such that both transistors conduct around the waveform crossoverpoint, thereby avoiding the crossover distortion characteristic of classB amplifiers. Each of the output transistors will be enabled for morethan one half cycle of an input signal, but much less than one fullcycle of the input signal. In this way, in a class AB amplifier, each ofthe push-pull transistors conducts for slightly more than the half cycleof conduction of class B, but much less than the full cycle ofconduction of class A. Unfortunately, the bias voltage applied to theoutput transistors in conventional AB class op-amps tends to reduce theheadroom (e.g., output swing) available at the output of the amplifierstage that drives the output transistors. Consequently, the gain of theamplifier stage, and the amount of resistive degeneration that can beused to reduce noise, are limited.

Embodiments of the present disclosure include novel class AB biasingcircuitry that increases the headroom available at the output of theamplifier stage that drives the output transistors. The increasedheadroom allows for higher gain in the amplifier stage and noisereduction due to use of increased resistive degeneration. The AB biasingcircuitry disclosed herein also increases the output transconductance ofthe op-amp, which improves the total harmonic distortion of the op-ampfor a given bandwidth and load, and allows for the use of a smallisolation resistor when driving a large capacitive load.

FIG. 1 shows a schematic diagram for a class AB output operationalamplifier (op-amp) 100 in accordance with various embodiments. Theop-amp 100 is a two-stage device that includes an input stage 102 and anoutput stage 104. The input stage 102 is electrically coupled to theoutput stage 104. In some embodiments, the op-amp 100 may include morethan two stages. For example, the input stage 102 may include multipleamplification stages. The input stage 102 receives input signals INP andINM and amplifies the difference of the input signals to produce anoutput X 106 of the input stage 102. The output X 106 of the input stage102 is provided to the output stage 104. The input stage 102 may includeany number of transistors arranged to form an amplifier. For example,the input stage 102 may include a cascode amplifier made up of two ormore stacked transistors.

The output stage 104 includes low side output transistor 112, high sideoutput transistor 114, low-side auxiliary amplifier 108, and high-sideauxiliary amplifier 110. The output transistors 112, 114 drive an outputterminal of the op-amp 100. The auxiliary amplifiers 108, 110 receive asinputs the output X 106 of the input stage 102 and predetermined voltageV_(mid), which may be equivalent to one-half of the power supply voltage(VDD) that powers the op-amp 100. The auxiliary amplifiers 108, 110 biasthe output transistors 112, 114 for class AB operation, and drive theoutput X 106 of the input stage 102 to V_(mid).

The auxiliary amplifiers 108, 110 isolate the input stage 104 from theclass AB bias voltages applied to the output transistors 112, 114.Consequently, the output X 106 is forced to V_(mid) voltage, which canbe set to mid-supply, in the auxiliary amplifiers 108, 110. By isolatingthe input stage 102 from the class AB bias voltage, the input stage 102is provided with more headroom than is provided in a conventional op-ampthat does not isolate the stage driving the output stage from the biasvoltage. The term headroom refers to the voltage range over which theoutput 106 can swing without causing any of the metal oxidesemiconductor (MOS) transistors to go into linear region. As a result ofthe increased headroom, the gain of the input stage 102 can beincreased, and increased resistive degeneration applied to provide noisereduction, relative to conventional op-amps.

The auxiliary amplifiers 108, 110 also increase the transconductance ofthe output stage 104, which in turn improves the total harmonicdistortion of the op-amp 100 for a given bandwidth and load. Theincreased transconductance of the output stage 104 also allows for useof a small isolation resistor when drive large capacitive loads with theop-amp 100.

FIG. 2 shows a schematic diagram for the low side auxiliary amplifier108 in accordance with various embodiments. The low side auxiliaryamplifier 108 includes transistors 202 and 204 arranged as adifferential pair. An input terminal (e.g., the gate terminal) oftransistor 202 is connected to the output X 106 of the input stage 102.An input terminal (e.g., the gate terminal) of the transistor 204 isconnected to the predetermined voltage V_(mid). The low side auxiliaryamplifier 108 also includes a transistor 206 that is arranged to operateas a diode and connected to the output terminal 210 of the auxiliaryamplifier 108 via a resistor 208. This device 206 is matched to thelow-side output device 112 and sets the quiescent current in the outputbranch of the main amplifier 100. In FIG. 2, the transistor 206 has awidth and length of Wn/Ln and is biased with current Ib. The low-sideoutput transistor 112 is scaled by M, resulting on a quiescent outputcurrent of M*Ib. The gain (A₁) of the low-side auxiliary amplifier 108may be determined as:

A ₁ =gm ₃*(R+1/gm ₄)

where:gm₃ is the transconductance of transistors 202 and 204;gm₄ is the transconductance of the transistor 206; andR is the resistance of resistor 208.

FIG. 3 shows a schematic diagram for the high-side auxiliary amplifier110 in accordance with various embodiments. The high-side auxiliaryamplifier 110 includes transistors 302 and 304 arranged as adifferential pair. An input terminal (e.g., the gate terminal) oftransistor 302 is connected to the output X 106 of the input stage 102.An input terminal (e.g., the gate terminal) of the transistor 304 isconnected to the predetermined voltage V_(mid). The auxiliary amplifier110 also includes a transistor 306 that is arranged to operate as adiode and connected to the output terminal 310 of the auxiliaryamplifier 110 via a resistor 308. This device 306 is matched to thehigh-side output device 114 and sets the quiescent current in the outputbranch of the main amplifier 100. In FIG. 3, the transistor 306 has awidth and length of Wn/Ln and is biased with current Ib. The high-sideoutput transistor 114 is scaled by M resulting on a quiescent outputcurrent of M*Ib. The gain (A₂) of the high side auxiliary amplifier 110may be determined as:

A ₂ =gm ₅*(R+1/gm ₆)

where:gm₅ is the transconductance of transistors 302 and 304;gm₆ is the transconductance of the transistor 306; andR is the resistance of resistor 308.

FIG. 4 shows a schematic diagram 400 for a model of the class AB outputop-amp 100 in accordance with various embodiments. The model includestwo output paths to reflect the two output transistors 112, 114 and thecorresponding two auxiliary amplifiers 108, 110. In the model, thetransconductance of the output stage 104 may be determined as:

gm _(o) =A ₁ *gm _(n) +A ₂ *gm _(p)

where:gm_(n) is the transconductance of output transistor 112;gm_(p) is the transconductance of output transistor 114; andA₁ and A₂, the gains of the auxiliary amplifiers 108, 110, are less than10.

In some embodiments of the op-amp 100, the gains A₁ and A₂ may be large.Such embodiments may include nested miller compensation.

FIG. 5 shows a model 500 of the class AB op-amp 100 in accordance withvarious embodiments for use in pole-zero analysis. The model 500 hasbeen simplified relative to the model 400 by assuming that the twooutput paths of the model 400 are equivalent and can be merged into asingle path. In this model, if C₂>>C_(L), the output pole is real andP₂˜gm₂*R*gm_(o)/C_(L). In general, the output pole is a complexconjugate pole pair with pole frequency ω_(o) and quality factor Q,where:

${\omega_{o} \approx \sqrt{\frac{{gm}_{2}*{gm}_{o}}{C_{2}*C_{L}}}},\mspace{14mu} {and}$${Q \approx {R*C_{2}{\sqrt{\frac{{gm}_{2}*{gm}_{o}}{C_{2}*C_{L}}}.}}}\mspace{20mu}$

FIG. 6 shows another model 600 of a class AB op-amp in accordance withvarious embodiments used for pole-zero analysis. The model 600 issimilar to the model 500 with an additional resistor R_(S) and capacitorC_(o). In the model 600, C_(o)>>C_(L) or C₂. C_(o) may typically beabout 10 nano-farads. There is a pole-zero pair in the unity gainbandwidth:

˜1/(2πR _(S) C _(o))

There is also a complex conjugate pole pair with frequency ω_(o) andquality factor Q.

${\omega_{o} \approx \sqrt{\frac{{gm}_{2}*{gm}_{o}}{C_{2}*C_{L}} + \frac{1}{R*R_{S}*C_{2}*C_{L}}}},\mspace{14mu} {and}$$\frac{\omega_{o}}{Q} \approx {\frac{1}{R*C_{2}} + {\frac{1}{R_{S}*C_{L}}.}}$

While embodiments of the op-amp 100 have been illustrated using metaloxide semiconductor field effect transistors, some embodiments mayinclude bipolar transistors, such as bipolar junction transistors, orother transistors of various technologies to implement the variousembodiments of the op-amp 100.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. An operational amplifier, comprising: an outputstage comprising a first output transistor and a second outputtransistor; an input stage configured to drive the output stage; a firstauxiliary amplifier coupled to an output of the input stage and an inputof the first output transistor, the first auxiliary amplifier configuredto bias the first output transistor for class AB operation and toisolate the input stage from a bias voltage applied to the first outputtransistor; a second auxiliary amplifier coupled to the output of theinput stage and an input of the second output transistor, the secondauxiliary amplifier configured to bias the second output transistor forclass AB operation and to isolate the input stage from a bias voltageapplied to the second output transistor.
 2. The operational amplifier ofclaim 1, wherein the first auxiliary amplifier comprises a first inputtransistor and a second input transistor coupled as a differential pair,wherein an input terminal of the first input transistor is coupled tothe output of the input stage, and an input terminal of the second inputtransistor is connected to a predetermined DC voltage.
 3. Theoperational amplifier of claim 2, wherein the first auxiliary amplifiercomprises a third transistor configured as a diode, wherein the thirdtransistor is connected to a common and connected, via a resistor, to anoutput of the first auxiliary amplifier, to set a quiescent outputcurrent of the first output transistor.
 4. The operational amplifier ofclaim 3, wherein the first auxiliary amplifier provides gain as afunction of transconductance of the first input transistor and thesecond input transistor and a sum of resistance of the resistor andreciprocal of transconductance of the third transistor.
 5. Theoperational amplifier of claim 1, wherein the second auxiliary amplifiercomprises a first transistor and a second transistor coupled as adifferential pair, wherein an input terminal of the first inputtransistor is coupled to the output of the input stage, and an inputterminal of the second input transistor is connected to a predeterminedDC voltage.
 6. The operational amplifier of claim 2, wherein the secondauxiliary amplifier comprises a third transistor configured as a diode,wherein the third transistor is connected to a power supply voltage andconnected, via a resistor, to an output of the second auxiliaryamplifier, to set a quiescent output current of the second outputtransistor.
 7. The operational amplifier of claim 4, wherein the firstauxiliary amplifier provides gain as a function of transconductance ofthe first input transistor and the second input transistor and a sum ofresistance of the resistor and reciprocal of transconductance of thethird transistor.
 8. The operational amplifier of claim 1, whereintransconductance of the output stage is approximately a sum of: aproduct of gain of the first auxiliary amplifier and transconductance ofthe first output transistor; and a product of gain of the secondauxiliary amplifier and transconductance of the second outputtransistor.
 9. The operational amplifier of claim 8, wherein the gain ofthe first auxiliary amplifier is less than ten and the gain of thesecond auxiliary amplifier is less than ten.
 10. The operationalamplifier of claim 1, wherein headroom of the first stage isapproximately one-half of the power supply voltage that powers theoperational amplifier.
 11. A multi-stage operational amplifier,comprising: a first stage configured to amplify an input voltage; asecond stage configured to drive an output terminal of the two-stageoperational amplifier, wherein the second stage is coupled to an outputof the first stage; a first auxiliary amplifier coupled to the output ofthe input stage and to a first input of the second stage, the firstauxiliary amplifier configured to: generate a first bias voltage andapply the first bias voltage to the second stage for class AB operation;and isolate the first stage from the first bias voltage; and a secondauxiliary amplifier coupled to the output of the input stage and to asecond input of the second stage, the second auxiliary amplifierconfigured to: generate a second bias voltage and apply the second biasvoltage to the second stage for class AB operation; and isolate thefirst stage from the second bias voltage.
 12. The multi-stageoperational amplifier of claim 11, wherein each of the first auxiliaryamplifier and the second auxiliary amplifier comprises a first inputtransistor and a second input transistor coupled as a differential pair,and wherein in each of the first auxiliary amplifier and the secondauxiliary amplifier an input terminal of the first input transistor iscoupled to the output of the first stage, and an input terminal of thesecond input transistor is connected to a predetermined DC voltage. 13.The multi-stage operational amplifier of claim 12, wherein: the firstauxiliary amplifier comprises a third transistor configured as a diode,wherein the third transistor is connected to a common and connected, viaa resistor, to an output of the first auxiliary amplifier; and thesecond auxiliary amplifier comprises a fourth transistor configured as adiode, wherein the fourth transistor is connected to a power supplyvoltage and connected, via a resistor, to an output of the secondauxiliary amplifier.
 14. The multi-stage operational amplifier of claim13, wherein a gain of each of the first auxiliary amplifier and thesecond auxiliary amplifier is a function of transconductance of thefirst input transistor and the second input transistor and a sum ofresistance of the resistor and reciprocal of transconductance of one ofthe third transistor or the fourth transistor.
 15. The multi-stageoperational amplifier of claim 11, wherein transconductance of thesecond stage is approximately a sum of: a product of gain of the firstauxiliary amplifier and transconductance of a low-side outputtransistor; and a product of gain of the second auxiliary amplifier andtransconductance of a high-side output transistor.
 16. The multi-stageoperational amplifier of claim 15, wherein the gain of the firstauxiliary amplifier is less than ten and the gain of the secondauxiliary amplifier is less than ten.
 17. The multi-stage operationalamplifier of claim 11, wherein the first auxiliary amplifier and thesecond auxiliary amplifier provide headroom in the first stage ofapproximately one-half of the power supply voltage that powers thetwo-stage operational amplifier.
 18. A class AB output operationalamplifier, comprising: an output stage comprising a high-side outputtransistor and a low-side output transistor; an input stage configuredto drive the output stage; a first auxiliary amplifier coupled to anoutput of the input stage and an input of the low-side outputtransistor, the first auxiliary amplifier configured to bias thelow-side output transistor for class AB operation and to isolate theinput stage from a bias voltage applied to the low-side outputtransistor, the first auxiliary amplifier comprising: a first transistorand a second transistor coupled as a differential pair, wherein an inputterminal of the first transistor is coupled to the output of the inputstage, and an input terminal of the second transistor is connected to apredetermined DC voltage; a second auxiliary amplifier coupled to theoutput of the input stage and an input of the high-side outputtransistor, the second auxiliary amplifier configured to bias thehigh-side output transistor for class AB operation and to isolate theinput stage from a bias voltage applied to the high-side outputtransistor, the second auxiliary amplifier comprising: a thirdtransistor and a fourth transistor coupled as a differential pair,wherein an input terminal of the third transistor is coupled to theoutput of the input stage, and an input terminal of the fourthtransistor is connected to the predetermined DC voltage; whereinisolation of the input stage from the output stage by the firstauxiliary amplifier and the second auxiliary amplifier provides theinput stage with headroom of approximately one-half of the power supplyvoltage that powers the operational amplifier.
 19. The class AB outputoperational amplifier of claim 18, wherein: the first auxiliaryamplifier comprises a fifth transistor configured as a diode, whereinthe fifth transistor is connected to a common and connected, via a firstresistor, to an output of the first auxiliary amplifier; the firstauxiliary amplifier provides gain as a function of transconductance ofthe first transistor and the second transistor and a sum of resistanceof the first resistor and reciprocal of transconductance of the fifthtransistor; the second auxiliary amplifier comprises a sixth transistorconfigured as a diode, wherein the sixth transistor is connected to apower supply voltage and connected, via a second resistor, to an outputof the second auxiliary amplifier; the second auxiliary amplifierprovides gain as a function of transconductance of the third transistorand the fourth transistor and a sum of resistance of the second resistorand reciprocal of transconductance of the sixth transistor.
 20. Theclass AB output operational amplifier of claim 19, whereintransconductance of the output stage is approximately a sum of: aproduct of gain of the first auxiliary amplifier and transconductance ofthe low-side output transistor; and a product of gain of the secondauxiliary amplifier and transconductance of the high-side outputtransistor.